1. Field of the Invention
The present invention relates to acoustic wave filter devices which utilize surface acoustic waves and boundary acoustic waves, and more particularly, to an acoustic wave filter device and a duplexer including the acoustic wave filter, each of which includes a plurality of longitudinally coupled resonator type surface acoustic wave filters connected to one another.
2. Description of the Related Art
In general, cellular phones require a reduction in the number of components included therein in order to reduce the size thereof. Therefore, it is desirable that a single component has a plurality of functions. An example of such a component having a plurality of functions is a balanced duplexer. The balanced duplexer includes a transmission filter connected to an antenna terminal and a reception filter having a balanced-to-unbalanced conversion function. Since the reception filter includes the balanced-to-unbalanced conversion function, a component providing the balanced-to-unbalanced conversion function, that is, a balun, can be eliminated.
An example of a surface acoustic wave filter device used as a reception filter device is disclosed in Japanese Unexamined Patent Application Publication No. 2002-290203.
FIG. 19 is a plan view schematically illustrating a configuration of the electrodes of the surface acoustic filter device disclosed in Japanese Unexamined Patent Application Publication No. 2002-290203.
A surface acoustic wave filter device 1001 is a balanced longitudinally coupled resonator type surface acoustic wave filter device. The surface acoustic wave filter device 1001 includes an electrode configuration provided on a piezoelectric substrate as shown in FIG. 19.
The surface acoustic wave filter device 1001 includes an unbalanced signal terminal 1002 defining an input terminal and first and second balanced signal output terminals 1003 and 1004 defining output terminals. First and second longitudinally coupled resonator type surface acoustic wave filters 1005 and 1006 are connected to the unbalanced signal terminal 1002. The first longitudinally coupled resonator type surface acoustic wave filter 1005 includes three interdigital transducers (IDTs) 1005a to 1005c and the second longitudinally coupled resonator type surface acoustic wave filter 1006 includes three IDTs 1006a to 1006c. Note that a region including the IDTs 1005a to 1005c is sandwiched between reflectors 1005d and 1005e in a surface wave propagation direction. In the second longitudinally coupled resonator type surface acoustic wave filter 1006, reflectors 1006d and 1006e are similarly arranged.
One end of the center IDT 1005b and one end of the center IDT 1006b are connected to the unbalanced signal terminal 1002, and the other end of the IDT 1005b and the other end of the IDT 1006b are connected to a ground potential. One end of the IDT 1005a and one end of the IDT 1005c arranged on both sides the center IDT of the first longitudinally coupled resonator type surface acoustic wave filter 1005, respectively, are connected to the first balanced signal terminal 1003. The other end of the IDT 1005a and the other end of the IDT 1005c are connected to the ground potential. Similarly, one end of the IDT 1006a and one end of the IDT 1006c arranged on both sides the center IDT of the first longitudinally coupled resonator type surface acoustic wave filter 1006, respectively, are connected to the second balanced signal terminal 1004. The other end of the IDT 1006a and the other end of the IDT 1006c are connected to the ground potential.
Here, a balanced-to-unbalanced conversion function is obtained such that a phase of a signal output from the first longitudinally coupled resonator type surface acoustic wave filter 1005 in response to a signal input to the first longitudinally coupled resonator type surface acoustic wave filter 1005 differs by about 180 degrees with respect to a phase of a signal output from the second longitudinally coupled resonator type surface acoustic wave filter 1006 in response to a signal input to the second longitudinally coupled resonator type surface acoustic wave filter 1006. Design parameters of the first longitudinally coupled resonator type surface acoustic wave filter 1005 are substantially the same as those of the second longitudinally coupled resonator type surface acoustic wave filter 1006, except for the phase of the output signal relative to the input signal.
Since the longitudinally coupled resonator type surface acoustic wave filter device 1001 disclosed in Japanese Unexamined Patent Application Publication No. 2002-290203 includes the balanced-to-unbalanced conversion function, the surface acoustic wave filter device 1001 can be used as the reception filter of the balanced duplexer described above. Accordingly, a reduction in the number of components of a cellular phone can be achieved.
However, in a reception filter of a duplexer, an attenuation in a passband of a transmission filter should be sufficiently large. When the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 2002-290203 is used as the reception filter, the amount of attenuation in the passband of the transmission filter, that is, an attenuation outside of a passband of the surface acoustic wave filter device 1001 defining the reception filter should be sufficiently large.
In typical cellular phones, a transmission frequency band is lower than a reception frequency band. Accordingly, when the surface acoustic wave filter device 1001 is used as the reception filter, an attenuation in frequencies which are outside of the passband and which is lower than the passband should be sufficiently large.
In a typical longitudinally coupled resonator type surface acoustic wave filter device, an attenuation in frequencies lower than a passband significantly depends on the frequency characteristics of the IDTs, and is primarily determined by synthesizing the frequency characteristics of all of the IDTs. That is, when an attenuation in a specific frequency band is large, attenuation poles of frequency characteristics of a plurality of IDTs substantially correspond to each other, whereas when an attenuation in a wide frequency band is made to be large, attenuation poles of frequency characteristics of a plurality of IDTs are slightly shifted from one another. However, the number of pairs of electrode fingers of the IDTs significantly affects a passband width and impedance characteristics, and accordingly, a degree of freedom of control is restricted.
On the other hand, when an attenuation outside of the passband is made to be large, the number of pairs of electrode fingers of the IDTs in the first longitudinally coupled resonator type surface acoustic wave filter 1005 may be different from the number of pairs of electrode fingers of the IDTs in the second longitudinally coupled resonator type surface acoustic wave filter 1006. However, when the amount of out-of band attenuation is increased by setting the number of pairs of electrode fingers of the IDTs in the first longitudinally coupled resonator type surface acoustic wave filter 1005 to be different from the number of pairs of electrode fingers of the IDTs in the second longitudinally coupled resonator type surface acoustic wave filter 1006, a problem arises in that a degree of balance between signals output from the first and second balanced signal output terminals 1003 and 1004 is significantly degraded.
Recently, instead of a surface acoustic wave filter device, a boundary acoustic wave filter utilizing boundary acoustic waves has been disclosed. The boundary acoustic wave filter has a simple package configuration. Similar to the surface acoustic wave filter, there are demands for a boundary acoustic wave filter having a balanced-to-unbalanced conversion function and a configuration capable of increasing an out-of-band attenuation.